Dispersible hydrophilic polymer compositions for use in viscosifying heavy brines

ABSTRACT

A thickening composition for use in viscosifying aqueous mediums, particularly heavy brines, comprising hydroxyethyl cellulose, 2-pyrrolidone and a mono alkyl ether of ethylene glycol which is water soluble and exerts no appreciable swelling effect on the hydroxyethyl cellulose, the weight ratio of hydroxyethyl cellulose to 2-pyrrolidone being less than about 2.6:1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to compositions for use as thickening orviscosifying agents in aqueous systems and, more particularly, theaqueous well servicing fluid prepared therefrom.

2. Description of the Background

It is known from U.S. Pat. Nos. 4,330,414 and 4,435,564 thatcompositions containing HEC can be used to thicken aqueous substancessuch as heavy brines commonly employed as well servicing fluids, if theHEC is formulated in certain ways. Thus, for example, in U.S. Pat. No.4,330,414, it is taught that a mixture of HEC, a solvating agent, whichis an agent that pre-hydrates the HEC, and a diluent which is anon-solvating agent for the HEC will effectively viscosify heavy brines,i.e. brines having a density greater than about 11.7 pounds per gallon(ppg) and more generally from about 12.0 ppg to about 19.2 ppg. U.S.Pat. No. 4,435,564 teaches that a composition containing HEC, certainamino compounds and an organic liquid which does not pre-hydrate the HECis useful in viscosifying heavy brines at ambient temperatures.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved composition for thickening aqueous mediums, especially heavybrines having a density greater than about 11.7 ppg.

A further object of the present invention is to provide an improved,aqueous well servicing fluid.

The above and other objects of the present invention will becomeapparent from the description given herein and the appended claims.

In accordance with the present invention, there is provided, in oneembodiment of the invention, a thickening composition for use inviscosifying aqueous mediums comprising HEC, 2-pyrrolidone and a monoalkyl ether of ethylene glycol which is water soluble and does notappreciably swell the HEC, the weight ratio of the HEC to the2-pyrrolidone being less than about 2.5:1.

In another embodiment of the present invention, there is provided a wellservicing fluid comprising an aqueous medium such as a brine and thethickening composition described above.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The thickening compositions of the present invention utilize, as ahydrophilic polymer, HEC as the primary component to provide theviscosifying effect. HEC polymers are solid, particulate materials whichare water soluble or water dispersible and which upon solution ordispersion in an aqueous medium increase the viscosity of the medium.HEC polymers are generally high yield, water soluble, non-ionicmaterials produced by treating cellulose with sodium hydroxide followedby reaction with ethylene oxide. Each anhydroglucose unit in thecellulose molecule has three reactive hydroxy groups. The average numberof moles of the ethylene oxide that becomes attached to eachanhydroglucose unit in cellulose is called moles of substituentcombined. In general, the greater the degree of substitution, thegreater the water solubility. In general, it is preferable to use HECpolymers having as high a mole substitution level as possible.

In addition to HEC, the compositions of the present invention alsocontain 2-pyrrolidone and a mono alkyl ether of ethylene glycol. Themono alkyl ethers of ethylene glycol useful in the compositions of thepresent invention are those ethers which are water soluble or watermiscible and exert no appreciable swelling effect on the HEC. Forpurposes of determining if a given ether is suitable for use in thecompositions of the present invention, the following test can be used:two parts by weight of the liquid ether is mixed with one part by weightof the HEC in a sealable container, and the mixture allowed to remain ina quiescent state in the sealed container for a period of about oneweek. If the ether being tested has no appreciable swelling effect onthe HEC, there will be free, liquid ether in the container after the oneweek period. Non-limiting examples of suitable ethers within the scopeof the present invention include ethylene glycol monobutyl ether(2-butoxyethanol)), ethylene glycol isobutyl ether(2-(2-butoxyethoxy)-ethanol), etc. Ethers such as ethylene glycolmonohexyl ether are unsuitable for use since they are insoluble inwater.

In the compositions of the present invention, the HEC will generally bepresent in a weight ratio of HEC polymer to 2-pyrrolidone of less thanabout 2.6:1, preferably in the range from about 2.6:1 to about 1:2.Desirable compositions, which are pourable liquids, and which willeffectively viscosify aqueous mediums can be produced from compositionscontaining from about 10 to about 25% by weight HEC polymer, from about10 to about 60% by weight 2-pyrrolidone and from about 10 to about 60%by weight of the mono alkyl ether of ethylene glycol.

A feature of the compositions of the present invention is that there isno necessity, as apparently taught in U.S. Pat. Nos. 4,330,414 and4,435,564, to utilize organophilic clay-like materials to formulatecertain well servicing fluids. Such clay containing materials areundesirable in clear well servicing fluids used for workover andcompletion operations. Also, since the thickening compositions of thepresent invention are essentially non-aqueous, problems associated withrusting of containers used to transport and store the thickening agentsare eliminated.

While the thickening compositions of the present invention are useful asviscosifiers or suspending agents in numerous systems which requireviscosity enhancement, they find particular utility in the preparationof well servicing fluids and, more particularly, well servicing fluidsmade from aqueous mediums containing soluble salts such as, for example,a soluble salt of an alkali metal, an alkaline earth metal, a Group IBmetal, a Group IIB metal, as well as water soluble salts of ammonia andother cations. The thickener compositions are particularly useful in thepreparation of thickened heavy brines, i.e. aqueous solutions of solublesalts of multivalent cations, e.g. Zn and Ca.

The most preferred thickened heavy brines, particularly those used forwell servicing fluids, i.e. completion and workover fluids, are madefrom brines having a density greater than about 11.7 ppg, particularlyfrom about 12 to about 19.2 ppg. Especially preferred heavy brinescomprise water solutions of salts selected from the group consisting ofcalcium chloride, calcium bromide, zinc chloride, zinc bromide, andmixtures thereof. Generally, in formulating well servicing fluids usingheavy brines, the thickening composition will be present in an amount offrom about 0.5 to about 3 ppg, calculated as active HEC.

To more fully illustrate the present invention, the followingnon-limiting examples are presented. In the following examples, the HECpolymer employed, unless otherwise indicated, was an HEC marketed byHercules under the tradename NATRASOL 250 HHR.

EXAMPLE 1

Thickening compositions were prepared by admixing together 2-pyrrolidoneand 2-butoxyethanol followed by the addition of HEC with stirring forapproximately five minutes. The thickening composition thus preparedcontained 38% by weight 2-pyrrolidone, 42% by weight 2-butoxyethanol and20% by weight HEC. The product was found to be pourable, fast yieldingand possessed excellent suspension characteristics, there being noliquid/solid separation.

The rheological properties of the thickening composition prepared abovewere evaluated at a concentration of 1.5 pounds per barrel, active HECin various brines. The data are shown in the table below.

                                      TABLE                                       __________________________________________________________________________               AV.sup.1 For Stir Time, Min                                                                 AV After Not                                         Test Brine 0  20 40  60  Roll 16 hr 150°                                                                % of max yield.sup.2                         __________________________________________________________________________    9.7 lb/gal KCl                                                                           1.5                                                                              20.5                                                                             21.5                                                                              22  21.5    102                                          10.0 lb/gal NaCl                                                                         2.5                                                                              26.5                                                                             31  32  33      97                                           11.3 lb/gal KBr                                                                          1.5                                                                              24 25  25  25.5    98                                           12.4 lb/gal NaBr                                                                         2.5                                                                              32.5                                                                             34  34.5                                                                              42.5    81                                           11.6 lb/gal CaCl.sub.2                                                                   8.5                                                                              61.5                                                                             70.5                                                                              72.5                                                                              78.5    92                                           14.2 lb/gal CaBr.sub.2                                                                   7  59 66  69  74.5    92                                           15.0 lb/gal CaBr.sub.2                                                                   12 62.5                                                                             83.5                                                                              89  98      91                                           16.0 lb/gal Ca/ZnBr.sub.2                                                                9  27 36.5                                                                              43  87.5    49                                           19.2 lb/gal Ca/ZnBr.sub.2.sup.3                                                          31 89 132 164 221     74                                           __________________________________________________________________________     .sup.1 All samples stirred 60 min. using Fann 35A rheometer at 600 RPM,       75° F.                                                                 .sup.2 % maximum yie1d: (AV after 1 hr. at 600 RPM/AV after hot ro11)         × 100                                                                   .sup.3 Data for 19.2 lb/gal Ca/ZnBr.sub.2 system based on 300 RPM dial        readings                                                                 

The percent of maximum yield is an indication of the ease with which thebrine can be viscosified without shearing or heat. As can be seen, evenin very heavy brines, the compositions of the present invention provideexcellent viscosification without the necessity for shearing or elevatedtemperatures.

EXAMPLE 2

A thickening composition containing 30% by weight 2-pyrrolidone, 50% byweight 2-butoxyethanol, and 20% HEC was prepared. The formulationexhibited rheological properties similar to the thickening compositionprepared as per Example 1. Pour characteristics of the thickeningcomposition were found to be slightly better than the thickeningcomposition produced by the procedure of Example 1.

EXAMPLE 3

To assess the effect of long term storage on the thickening compositionof the present invention, an accelerated aging study was conducted. Twothickening compositions prepared in accordance with the procedure ofExample 1 were crimp sealed in five gallon, unlined, mild steel pailsplaced in a 120° F. steam bath heated hot house for three months. Afterthree months aging, the steel pails were in excellent condition, norusting or deterioration being observed. Although the thickeningcompositions were found to be marginally pourable, the rheologicalcharacteristics of the thickening composition were unaffected.

EXAMPLE 4

The procedure of Example 1 was followed with the exception that2-(2-butoxyethoxy)-ethanol was substituted for 2-butoxyethanol. Althoughacceptable, the rheological property of the thickening compositioncontaining the 2-isobutoxyethanol were not as good as those obtainedwith the thickening composition of Example 1.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which some within the meaning and range ofequivalence of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A composition for use in viscosifying aqueousmediums comprising hydroxyethyl cellulose, 2-pyrrolidone and a monoalkyl ether of ethylene glycol which is water soluble and which has noappreciable swelling effect on the hydroxyethyl cellulose, the weightratio of hydroxyethyl cellulose to 2-pyrrolidone being less than about2.6:1, said 2-pyrrolidone being present in an amount of from about 10 toabout 60% by weight and said ether is present in an amount of from about10 to about 60% by weight.
 2. The composition of claim 1 wherein theweight ratio of said hydroxyethyl cellulose to said 2-pyrrolidone isfrom about 2.6:1 to about 1:2.
 3. The composition of claim 1 whereinsaid hydroxyethyl cellulose is present in an amount of from about 10 toabout 25% by weight.
 4. The composition of claim 1 wherein said monoalkyl ether comprises 2-butoxyethanol.
 5. A well servicing fluidcomprising:an aqueous medium; and a viscosifying composition comprisinghydroxyethyl cellulose, 2-pyrrolidone and a mono alkyl ether of ethyleneglycol which is water soluble and which has no appreciable swellingeffect on the hydroxyethyl cellulose, the weight ratio of hydroxyethylcellulose to 2-pyrrolidone being less than about 2.6:1, wherein said2-pyrrolidone is present in an amount of from about 10 to about 60% byweight and said ether is present in an amount of from about 10 to about60% by weight.
 6. The composition of claim 5 wherein said aqueous mediumcomprises a solution of at least one water soluble salt of a multivalentmetal ion.
 7. The composition of claim 6 wherein said aqueous medium hasa density greater than about 11.7 pounds per gallon.
 8. The compositionof claim 7 wherein said water soluble salt is selected from the groupconsisting of calcium chloride, calcium bromide, zinc chloride, zincbromide and mixtures thereof.
 9. The composition of claim 8 wherein thedensity of said aqueous medium is from about 12.0 pounds per gallon toabout 19.2 pounds per gallon.
 10. The composition of claim 5 wherein theweight ratio of said hydroxyethyl cellulose to said 2-pyrrolidone isfrom about 2.6:1 to about 1:2.
 11. The composition of claim 5 whereinsaid hydroxyethyl cellulose is present in an amount of from about 10 toabout 25% by weight.
 12. The composition of claim 5 wherein said monoalkyl ether comprises 2-butoxyethanol.